1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to wireless communications.
2. Description of the Related Art
In the field of wireless telecommunications, such as cellular telephony, a typical system 100, as shown in FIG. 1, includes a plurality of base stations 130 (e.g., Node Bs) distributed within an area to be serviced by the system. Various Access Terminals 120 (ATs, also known as User Equipment (UE), mobile devices, and the like) within the area may then access the system and, thus, other interconnected telecommunications systems, such as a publicly switched telephone system (PSTN) 160 and a Data network 125, via one or more of the base stations 130. Typically, an AT 120 maintains communications with the system 100 as it passes through an area by communicating with one and then another base station 130, as the AT 120 moves. The AT 120 may communicate with the closest base station 130, the base station 130 with the strongest signal, the base station 130 with a capacity sufficient to accept communications, etc. The base stations 130, in turn, communicate with a Radio Network Controller (RNC) 138, which communicates with a core network 165. Each RNC 138 is capable of supporting a plurality of base stations 130.
In systems employing Universal Mobile Telephone System (UMTS) Long Term Evolution (LTE), it has been proposed that Orthogonal Frequency-Division Multiple Access (OFDMA) be employed for the uplink and downlink multiple access scheme. OFDMA has very high side-lobes due to the use of the Discrete Fourier Transform as the orthogonal set for the frequency domain modulation or multiple access control. These high side-lobes substantially reduce Out Of Band Emissions (OOBE) and spurious emissions, which is very beneficial in the LTE.
While a transmit power shaping filter, such as the one specified in TS 25.104, Base Station (BS) Radio Transmission and Reception (FDD), 3GPP, can meet the spectrum emission requirement, R1-051203, Windowing and spectral containment for OFDM downlink, LTE contribution, Lucent Technologies has shown that a better alternative is though a windowing function. Instead of filtering the transmitted signal, i.e., performing convolution in time domain, the windowing processing function multiplies the transmitted signal with a well-designed time sequence. Each OFDM symbol, plus the attached cyclic-prefix (CP) is multiplied in point-to-point fashion by a shaping sequence (windowing). This shaping function is chosen such that it has a short transition period at the beginning and end and remains constant in the middle. It reduces the OOBE at a small cost of effective CP reduction. This process is illustrated in FIG. 2.
The benefit of the windowing function is not only its implementation simplicity, but it also has a reduced guardband to meet out-of-band emission requirement. The reduced guardband increases the spectrum efficiency, especially in a wider bandwidth system. FIG. 3 shows 5 M, 10 M and 20 M Hz bandwidth OFDMA systems, using a window function of about 3.6% provides the amount of guardband required in UMTS.
Note that, with the same amount of windowing, required guardband does not increase with bandwidth. Thus, counting the cost on CP reduction, the overheads due to spectrum containment for 5 M, 10 M and 20 M Hz OFDM systems are around 13.2%, 8.4% and 6.0%. This compares favorably to a typical overhead of 25% for a single-carrier CDMA system and offers a significant spectrum efficiency advantage. FIG. 4 shows the spectrum of a typical CDMA system.
It should be noted that, windowing works well for wide band systems, but for narrower bands, e.g., 1.25 M Hz bandwidth system, windowing alone is not enough. Further filtering may be needed to shape the spectrum.
While such windowing function allows less guardband and thus increases spectrum efficiency on the transmission side, it does impose significant challenges to the receiver design, mainly in term of the required channel selectivity. Channel selectivity is the ability of a receiver to extract a signal from its own band and reject a signal on an adjacent band. By having less guardband on the transmission side, not only is interfering power on the adjacent band moved closer to the receiver's own band, but additionally, the receiver has to extend its filter pass-band as well to match the expanded information bandwidth from the transmitter. Thus, from the perspective of a receiver filter, not only is the attenuation requirement increased (although by a moderate amount), but also, the allowed transition band is significantly shortened. This puts a significant challenge on the receiver filter design, especially for cost-sensitive mobile units.